A CTP (Copy-to-Plate) system which is inexpensive and easy in handling and has printability equivalent to PS plates (presensitized litho plates) has been desired along with digitization of printing data.
In particular, there recently have been increased expectations for a so-called process-less plate which needs no processing with specific chemicals and is applicable to printing machines provided with direct imaging (also denoted simply as DI) functions.
However, such a process-less plate is offered substantially only for use in DI printing machines under present circumstances and any process-less plate exhibiting sufficient performance as a general purpose printing plate material has not been provided.
One reason that the process-less plate is still insufficient for use as a general-purpose printing plate material concerns its low image visibility after exposure (hereinafter, also denoted as exposure visibility).
In direct image printing machines, position proofing is substantially not conducted during the period that a printing plate material is set on a plate cylinder and imagewise exposed, then, printing is conducted, so that even in cases of low exposure visibility, no serious problem occurs. However, in cases when used as a general purpose printing plate material, position proofing is needed even when image-forming in a CTP system, requiring exposure visibility.
The main trend in process-less plates is a so-called thermal type using infrared laser exposure to perform image formation. This thermal type is mainly classified into two types.
One of such thermal type printing plate materials is a ablation type, in which two layers differing in affinity for an aqueous dampening liquid or ink used in printing are provided on a substrate and the layer on the surface side (hereinafter, also denoted simply as surface layer) is ablated by laser light exposure to perform complete removal.
In such a type, the foregoing two layer differing in color under visible light can provide exposure visibility and examples thereof include a printing plate material disclosed in published Japanese translations of PCT international publication for patent applications No. 2002-514984.
However, this type of printing plate material needs to provide a suction mechanism to completely remove flying pieces on the ablated surface layer within the exposure apparatus, producing problems such that apparatus costs greatly increase. Further, exposure requiring relatively high energy necessitates lowering a beam line speed at the time of exposure (for example, reducing the rotation speed of a exposure drum), leading to reduced productivity of image formation.
The other thermal type printing plate material is an development on press (or development on a printing machine). This type of printing plate material comprises two layers differing in affinity for an aqueous dampening liquid or ink used in printing on a substrate, in which laser light exposure is conducted to change adhesion between the surface layer and the under-layer and low-adhesive portions are removed on press. Removal of the low-adhesive portions can be carried out by contact with a dampening liquid-supplying roller, by dissolution or swelling by the supply of a dampening liquid, by contact with an ink roller, by peeling-off employing the ink tackiness property, or by contact with a blanket drum. At least a part of the surface layer is removed on press in the printing plate material of a type of development on press so that the layer to be removed is desired not to be colored to prevent color-staining of the dampening liquid or the ink of a printing machine. It is therefore difficult to provide superior visibility.
For example, JP-A No. 11-240270 (hereinafter, the term JP-A refers to Japanese patent application publication) disclosed a printing plate material comprising a heat-sensitive layer containing an infrared absorption dye and capable of change an optical density upon exposure to light, thereby providing visibility. However, although such infrared absorption dyes generally fade upon exposure to infrared rays, complete fade-away cannot be achieved. Accordingly, when exposed areas of the surface layer are removed, a few colored pieces are mixed with the dampening liquid or ink, causing stains. Further, in cases when unexposed areas of the surface layer are removed, it is necessary to enhance contrast between exposed and unexposed areas, that is, to increase the coloring density of the unexposed area to provide superior visibility, so that visibility and anti-staining of a printing machine are contrary in performance to each other.
For example, JP-A No. 2001-322226 discloses an image recording method to obtain contrast between exposed and unexposed areas of a development-on-press type printing plate material substantially without containing colored material in the layer to be developed on press, in which by using a planographic printing plate comprising a recording layer mainly containing hot melting particles and a porous layer containing voids on a support, the voids in the porous layer are filled with the thermally fused particles to record a contrast image of transparency and non-transparency.
Although this method achieves relatively favorable visibility without causing stains on the printing machine, disadvantages were such that since a light transmission property of the printing plate material was employed to achieve contrast formation, constant exposure visibility (e.g., difference in reflection density between exposed and unexposed areas) was not always achieved only by the printing plate material.